Under high speed conditions, the outer diameter of the tire increases due to centrifugal forces generated by steel cord belt package and tread. Such a diameter increase or tire growth increases the pantographic movements of the belt edge cords leading to the crack initiations, crack propagations and at the end belt edge separations.
On the other hand, the temperature rise at belt edges under high speed conditions might cause local adhesion degradation of the textile cap ply strip and leads to local cap ply-tread separations which may subsequently result in belt edge separations in a short time due to non-uniform stress distributions.
The cap ply layer wound on belt package circumferentially prevents excessive tire growth under high speed conditions by applying compressive forces (restraining force) on heavy belt package made of cross-ply steel cord layers. In order to enhance the restraining force, high cord count (epdm) cap ply strips are usually used (e.g. Nylon, 1400×2, 110 epdm or Nylon 1400×1, 140 epdm etc.).
Currently, most widely used cap ply materials are nylon 6.6 and aramid/nylon hybrid cords which are spirally wound on belt package at 0 to 5 degrees to equatorial plane of the tire.
Nylon cords have excellent fatigue resistance under bending and axial compression, and bielastic tensile characteristic enabling easy processing during tire building. Additionally, shrink force generation with increasing service temperature under high speed conditions enhances belt edge separation resistance and improves high speed durability.
It is well known that the hybrid cords comprising high and low modulus yarns having bi-elastic tensile behaviour are also used as cap ply in high speed tires. The low modulus component of hybrid cord enables easy belt package lifting without excessive tight cord formation due to its high extensibility and the high modulus component becomes effective in service conditions. By using hybrid cords as cap ply, the total thickness of the cap ply layer and rubber content can be decreased, and the high modulus component of the hybrid cord improves the restraining force and high speed durability.
U.S. Pat. No. 4,284,117 describes a nylon cap ply which is formed from single yarns rather than cords. The cap ply is thinner than conventional cap plies having superior flexibility and heat dissipation characteristics. But the limited fatigue resistance of the single ply cords is the primary drawback of this application. The twist increase to improve the fatigue resistance leads to drastic modulus and restraining force drop.
U.S. Pat. No. 5,115,853 a radial tire having a cap ply structure disposed radially outwardly of the belt reinforcing structure. The cap ply structure comprises side by side nylon cords of 420×2 having cord twist of not greater than 280 tpm. The limited twist, in this case, low twist (low helix angle) has negative effects on fatigue resistance and impact resistance.
U.S. Pat. No. 7,584,774 describes a polyethylene terephthalate (PET) cord as belt reinforcing layer (cap ply) spirally wound on belt package in circumferential direction. Said PET cord has an elastic modulus not less than 2.5 mN/dtex. % under a load 29.4N at 160° C. High modulus drop of PET cord at high speed temperatures limits the durability performance.